312 EDSON S. BASTIN 



portions showing high- and low-temperature characteristics are 

 frequently intimately associated in the same pegmatite mass, we have 

 here furnished a key to the general temperature of solidification of 

 many of these bodies, namely in the neighborhood of 560-80° C. 



Eutectics in pegmatites.- — Largely as a result of the extensive 

 studies of Vogt^ many geologists^ have been led to attribute an impor- 

 tant role to eutectics in rock formation. One of the first-^ phenom- 

 ena to suggest such a relation was obviously the graphic structure 

 exhibited by many pegmatites, which closely resembled patterns 

 formed by eutectic mixture in alloys. Vogt^ calculated the ratio 

 between quartz and feldspar in a number of analyses of graphic inter- 

 growths of quartz with microcline, the latter mineral being also 

 perthitically intergrown with various amounts of soda plagioclase. 

 The ratios were constant enough to lead Vogt to conclude that the 

 graphic granites represented eutectic mixtures. Slight disparities 

 between analyses he attributed to slight variations in the compositions 

 of the feldspars and to variations in the pressures under which the 

 granites had crystallized. In many cases, especially in microscopic 

 varieties, the graphic intergrowths are considered to be the end- 

 products of crystallization. 



In 1905, H. E. Johansson, -5 working mainly with Vogt's analyses, 

 computed the molecular proportions of the quartz and feldspars 

 present and arrived at the conclusion that these bore very simple 

 numerical relations to each other. In graphic granites with dominant 

 orthoclase the molecular ratio of feldspar to quartz was about 2:3. 

 In an oligoclase graphic granite the proportion was about 1:2, and 

 in an albite-quartz micropegmatite about 1:3. 



Later Bygden^ made a considerable number of other analyses 

 of graphic granites with the special purpose of determining to what 



' Vogt, Die Silikatschmelzlosungen, II, 117-35 (i9°3)- 



2 See Harker, The Natural History of Igneous Rocks, 262-66, 270-72. 



3 See Teall, British Petrography, 401-2 (1888). 



4 Op. cit., 120-21. 



5 H. E. Johansson, Geologiska joreningens jorhandlingar (Stockholm, 1905), 

 XXVII, 119. 



^ A. Bygden, "Ueber das quantitative VerhaUnis zwischen Feldspar und Quartz 

 in Schrift-Graniten," Bulletin of the Geological Institution oj the University of U psala, 

 VII, 1-18 (1904-5). 



